1. Field of the Invention
The present invention generally relates to electromagnetic transponders and, more specifically, an electromagnetic transponder with no autonomous power supply.
The present invention applies for example to smart cards or electronic tags communicating without any contact with a terminal and extracting the power necessary for the supply of the circuits that they comprise from the high-frequency electromagnetic field radiated by the terminal. The present invention especially applies to electronic tags or smart cards in applications of electronic passport type.
2. Description of the Related Art
Systems for electromagnetic transponders are based on the cooperation between an oscillating circuit on the read-write terminal side and a resonant circuit on the electromagnetic transponder side (generally, a portable element), to exchange information by using a high-frequency field radiated by the oscillating circuit of the terminal. In the transponders to which the present invention applies, the high-frequency carrier is also used as a remote-supply carrier providing the transponder supply power.
An example of application of the present invention relates to transponder systems based on standards ISO 14443 and 15693 according to which the remote-supply carrier radiated by the terminal is 13.56 MHz, while a back-modulation sub-carrier may be used by the transponders to transmit information to the terminal with a 847.5-kHz frequency. In the terminal-to-transponder transponder direction, the carrier is modulated in amplitude with a modulation factor generally on the order of 10%. The modulation factor defines as being the amplitude difference between the transmission of a state 1 and the transmission of a state 0, divided by the sum of these amplitudes. In fact, standards set a range of acceptable modulation factors that transponders are supposed to be able to interpret and that terminals are supposed to respect. This range, in the case of the above-mentioned standards, ranges between 8 and 14%.
FIG. 1 very schematically shows in the form of blocks an example of an electromagnetic transponder system to which the present invention applies. A transponder 1 (TR) is intended to be placed in the electromagnetic field of a terminal 2 (TERM) having an inductive element L2 of an oscillating circuit emitting a high-frequency radiation sensed by an antenna L1 of transponder 1.
FIG. 2 very schematically shows in the form of blocks an example of a conventional architecture of an electromagnetic transponder 1. The transponder comprises an oscillating circuit 10, formed of an inductive element L1 forming an antenna, in parallel with a capacitor Cl at the A.C. input terminals of a rectifying bridge 11. The rectified output terminals of bridge 11 are connected by a storage capacitor Cs. The signal sensed when transponder 1 is in the field of a terminal is used upstream and downstream of rectifying bridge 11.
Downstream, the rectified voltage recovered at the output of bridge 11 is especially used by a regulator 12 (REG) to extract a D.C. supply voltage Vdd of the transponder circuits. The output of bridge 11 is also connected to the input of a circuit 13 (DEM) for demodulating the received data, generally, an amplitude demodulator. Finally, a modulator 14 (MOD) of the information to be transmitted to the terminal by back modulation connects a microprocessor 15 (μP) or any other circuit of control and of digital interpretation of the transmissions, to a back-modulation stage connected to the output of bridge 11. For simplification, the back-modulation stage has been assumed to be integrated to block 14.
Upstream of rectifying bridge 11, the high-frequency signal is sampled toward, especially, a circuit 20 (CLK GEN) of generation of a clock from the high-frequency carrier and a circuit 21 (RF DET) of detection of the presence of a radio-frequency excitation across resonant circuit 10.
A disadvantage of conventional transponders is that supply voltage Vdd provided by regulator 12 is contaminated by noise due to the switchings performed downstream (especially by the microprocessor). This noise is propagated to the input of regulator 12 on the rectified signal of bridge 11, which adversely affects the signal-to-noise ratio of the system and thus makes the data extraction by demodulator 13 more difficult.
This problem is all the more present as the modulation factor is low. Now, a low modulation factor (typically, less than 20%) is further required to provide a sufficient remote-supply power. Indeed, an amplitude modulation transmission in all or nothing would divide approximately by two the remote-supply capacity.